1. Field of the Invention
This invention relates to a clock supplying apparatus for supplying a clock to a digital circuit.
2. Description of the Related Art
A digital circuit which uses a high speed clock signal is influenced much by jitters of the clock, and this influence is a cause in a drop in the operation speed or a malfunction. Therefore, the countermeasures as described below have been taken heretofore.
For a clock generation circuit, power supply lines are isolated from power supply lines for the other circuits in order to reduce jitters by power supply noise. Also, such countermeasures as the additional provision of a bypass controller, the insertion of a power supply filter or the use of a power supply regulator circuit for exclusive use have been taken in the past.
As a countermeasure against crosstalk noise on a clock transmission line, also, such countermeasures as laying the clock transmission line in a spaced relationship away from other signal lines or shielding the clock transmission line on the opposite sides with a ground (GND) signal line are known well.
As a different countermeasure, using an LC resonance circuit in a clock generation circuit as disclosed, for example, in Japanese Patent Laid-Open No. Hei 11-284439 (hereinafter referred to as Patent Document 1), is known.
The LC resonance circuit is provided additionally for a feedback loop for oscillation.
As a further method, using an LC resonance circuit in a reproduction circuit for a clock as disclosed, for example, in Japanese Patent Laid-Open No. Hei 1-256837 (hereinafter referred to as Patent Document 2) or Japanese Patent Laid-Open No. Hei 3-4633 (hereinafter referred to as Patent Document 3), is known.
The effects of an LC resonance circuit are obtained around a resonance frequency. Therefore, in order to make a plurality of clock frequencies available, one of a plurality of LC resonance circuits is used selectively or a variable inductor or a variable capacitor is used. A circuit of the type just described is disclosed, for example, in Japanese Patent Laid-Open No. Hei 3-266543 (hereinafter referred to as Patent Document 4) or Japanese Patent Laid-Open No. Hei 4-271575 (hereinafter referred to as Patent Document 5).